1. Field of the Invention
The present invention relates to an optical fiber. More particularly, the present invention relates to a dispersion-controlled fiber.
2. Description of the Related Art
In general, the dispersion characteristics of an optical fiber can be effectively controlled by positioning a region of a depressed refractive index between a core and a cladding of the fiber. This is disclosed in U.S. Pat. No. 4,715,679 (title: “LOW DISPERSION, LOW-LOSS SINGLE-MODE OPTICAL WAVEGUIDE”) invented by and issued to Venkata A. Bhagavatula, the contents of which are incorporated by reference as background material.
FIG. 1 is a graph illustrating prior art dispersion characteristics of a single-mode fiber (SMF). In this illustration, a dispersion curve 110 for the SMF is shown. The SMF has a step-index profile because there is no region having a depressed refractive index. As seen from the dispersion curve 110, the SMF has a unit dispersion value of about 17 ps/nm/km at a wavelength of 1550 nm. If the SMF is used for a long distance transmission, an accumulated dispersion of an optical signal received through the SMF is increased and, as a result, a distortion of the optical signal becomes more severe. There are various dispersion compensation techniques in the prior art for minimizing the accumulated dispersion occurring during the long distance transmission of the optical signal. Generally, a method of using a dispersion-controlled fiber has been widely employed to minimize the accumulated dispersion.
Dispersion-controlled fiber has a high negative dispersion value because of a depressed refractive index region surrounding its core. Further, the dispersion-controlled fiber can be connected to one end of the SMF to compensate for the accumulated dispersion of the SMF. The dispersion-controlled fiber has a high negative unit dispersion value at a wavelength of 1550 nm and its length may be adjusted to offset the accumulated dispersion of the SMF, so that the total dispersion becomes zero.
However, if the dispersion-controlled fiber is adapted for dispersion compensation of the SMF, a sum of an accumulated dispersion of the dispersion-controlled fiber and the accumulated dispersion of the SMF may not be zero at wavelengths other than 1550 nm. In this regard, there is a problem in which it is not appropriate to apply the dispersion-controlled fiber to a wavelength division multiplexing system.
In order to overcome the above problem, research has recently been done to provide a fiber capable of compensating for both a dispersion and a dispersion slope together. To compensate for both the dispersion and dispersion slope, it is required to let a dispersion value and dispersion slope of the SMF be DSMF and DSSMF and those of the dispersion-controlled fiber be DDCF and DSDCF, respectively, such that the DDCF and DSDCF satisfy the following equation 1.DSMF:DSSMF≅DDCF:DSDCF  [Equation 1]
If the dispersion and dispersion slope (DDCF and DSDCF) of the dispersion-controlled fiber satisfy equation 1, compensation for the accumulated dispersion of the SMF occurs not only at a wavelength of 1550 nm, but also at wavelengths other than 1550 nm. However, there is a great deal of difficulty implementing a fiber that perfectly satisfies equation 1 over the entire wavelength range. For this reason, the current state of the art simply compensates for the dispersion and dispersion slope at C-band wavelengths of 1530-1570 nm. In a wide band wavelength division multiplexing system, there is a need to perform the dispersion and dispersion slope compensations at any wavelength in a range of wavelengths including an S-band of 1450-1530 nm and L-band of 1570-1610 nm as well as the C-band.